1. Field of the Invention
The present invention relates generally to orthogonal frequency division multiplexed (OFDM)-based communications, and more specifically to tracking pilot tones of OFDM-based communications to reduce phase noise requirements in the radio portion of an OFDM receiver, as well as provide nearly optimal frequency error tracking performance.
2. Discussion of the Related Art
In wireless local area network (WLAN) applications, multiple devices communicate with each other via OFDM-based radio frequency (RF) wireless links. A common format for such OFDM communication is based upon the IEEE 802.11a standard or the HiperLAN2 standard, for example. Good local oscillator (LO) phase noise performance in the radio portion of the OFDM transmitters and receivers is critical in such OFDM-based communications when using complex signal constellations, such as 64-QAM and 256-QAM (quadrature amplitude modulation). This is because the symbol rate is chosen to be low enough to combat the severe multipath propagation characteristics that exist like those in indoor wireless applications and this low symbol rate also leads to greater phase noise related performance impairment. For example, in IEEE 802.11a and HiperLAN2, the symbol rate is approximately 250 kHz thereby accentuating the need to have excellent phase noise performance in the radio at frequency offsets from the carrier in the vicinity of 250 kHz and less.
Furthermore, the phase of the RF signaling is effected by phase noise generated in the local oscillators (LOs) of both the transmitter and the receiver. Also, phase perturbations are introduced when the transmitter or the receiver physically moves relative each other and also when the multipath changes, e.g., a door is opened. Unfortunately, poor LO phase noise performance leads to a potentially high symbol error rate, which seriously degrades both the communication range and throughput of the system. For example, in a typical system using IEEE 802.11a, it is estimated that the acceptable phase noise interfering with each subcarrier of the OFDM waveform is on the order of 2.7 degrees rms. While this may be acceptable for QPSK and 16-QAM modulations, it is excessive for 64-QAM modulation or higher constellations, resulting in constellation points being easily confused.
Further adding to the problem is the fact that most transmitters and receivers of such wireless products are highly integrated on a single device or chip. As such, the performance of the RF portion of the receiver, for example, is relatively limited. Furthermore, implementing the RF portion of the system to have the desired good phase noise performance that is required for higher order modulations, such as 64-QAM and above, is very difficult when implemented on a single chip with low supply voltages (e.g., 3.3 volts).
The present invention advantageously addresses the needs above as well as other needs by providing a pilot tracking system utilizing an optimum pilot phase error metric based on a maximum likelihood estimation approach in the baseband processing portion of the OFDM-based receiver to compensate for poor local oscillator performance in the radio portion of the OFDM-based receiver and transmitter and improve frequency tracking in general.
In one embodiment, the invention can be characterized as a pilot phase tracking loop for an orthogonal frequency division multiplexed (OFDM) receiver comprising a phase rotator for receiving an incoming signal, a Fourier transform coupled to an output of the phase rotator and a pilot phase error metric coupled to an output of the Fourier transform for determining a phase error estimate associated with a received OFDM symbol. A loop filter is coupled to an output of the pilot phase error metric and an oscillator is coupled to an output of the loop filter. The oscillator has an output coupled to the phase rotator for causing the phase rotator to rotate the incoming signal by the filtered phase error estimate for subsequent OFDM symbols such that the phase noise of the signaling output from the phase rotator is reduced.
In another embodiment, the invention can be characterized as a pilot phase tracking loop for an orthogonal frequency division multiplexed (OFDM) receiver comprising a phase rotator for receiving and adjusting the phase of an incoming signal, a Fourier transform coupled to an output of the phase rotator and a pilot phase error metric coupled to an output of the Fourier transform. Also, a loop filter coupled to the pilot phase error metric and an oscillator coupled to the loop filter and having an output coupled to the phase rotator.
In yet another embodiment, the invention can be characterized as a method for tracking pilot phase in an orthogonal frequency division multiplexed (OFDM) receiver comprising the steps of: receiving an incoming signal corresponding to an OFDM preamble waveform at a Fourier transform of the OFDM receiver; determining pilot reference points corresponding to a plurality of pilots of an OFDM preamble waveform; receiving an incoming signal corresponding to an OFDM symbol at the Fourier transform; determining complex signal measurements corresponding to each of the plurality of pilots of the OFDM symbol; determining a phase error estimate corresponding to the OFDM symbol; filtering the phase error estimate; and rotating a phase of the incoming signal for subsequent OFDM symbols to be received at the Fourier transform after the OFDM symbol by the filtered phase error estimate, wherein a phase noise of the incoming signal for the subsequent OFDM symbols is reduced.
In a further embodiment, the invention can be characterized as a method of pilot phase tracking in an orthogonal frequency division multiplexed (OFDM) receiver comprising the steps of: receiving an incoming signal representing an OFDM waveform at a Fourier transform of the OFDM receiver; determining a phase error estimate corresponding to an OFDM symbol of the OFDM waveform based upon the output of the Fourier transform; filtering the phase error estimate; and rotating a phase of the incoming signal for subsequent OFDM symbols to be received at the Fourier transform after the OFDM symbol by the filtered phase error estimate, wherein the phase noise of the incoming signal for the subsequent OFDM symbols is reduced.